Pull clip info from crossfades/fades via python
You can hover over an Effect strips Input boxes and get the Python variable:



So you just need to call .input_1 on the effect strip to access the first effected strip. Then you can access its attributes like normal (e.g. .filepath) as it's just a reference to the original strip.

The best help is your autocompletion (Ctrl+Space) in an interactive Python console view. So just write the complete bpy.data.scenes['Scene'].sequence_editor.sequences_all['Cross'].input_1 into one and hit the . (dot), then call the autocompletion and it will print you any possible accessible attributes of this strip.